1. Field of the Invention
The present invention relates to an apparatus for adjusting a contrast ratio of a liquid crystal display device, and more particularly to an apparatus for adjusting a contrast ratio of a liquid crystal display device which includes an analog voltage generator in order to adjust the contrast ratio of the liquid crystal display device.
2. Description of the Prior Art
As generally known in the art, when manufacturing a display device using a liquid crystal display device, a contrast ratio among various features is adjusted in order to control image quality of the display device.
Contrast ratio is a ratio between an ability displaying a screen most brightly and an ability displaying the screen most darkly and is related to the sharpness of the screen.
The contrast ratio is expressed by the following equation.CONTRAST RATIO=MAXIMUM BRIGHTNESS(cd/m2)/MINIMUM BRIGHTNESS(cd/m2)  [Equation 1]
For example, when the brightnesses which a liquid crystal display device displays most brightly and most darkly are 250 cd/m2, and 0.5 cd/m2, the contrast ratio of the liquid crystal display device is expressed as 250/0.5 (cd/m2) by using the equation 1.
FIG. 1 is a graph showing a method which adjusts a contrast ratio. That is, FIG. 1 shows a relationship between a voltage inputted to the liquid crystal display device and a screen brightness of a liquid crystal panel according to the input voltage.
The longitudinal axis indicates the voltage which is inputted to the liquid crystal display device. Reference numeral G (Gain) represents voltage width when embodying maximum and minimum brightnesses. The transverse axis indicates a brightness according to a change of the corresponding transmission factor of a liquid crystal panel when the voltage of the longitudinal axis changes by G width based on a Voffset voltage.
It is assumed that a Voffset1 voltage is set to a minimum brightness voltage. Accordingly, in the case of adjusting G width, the brightness of the screen varies within the range from 0% to 100% as shown in the longitudinal axis. At this time, when it is assumed that the screen brightness of 0% is 0.5 cd/m2 and the screen brightness of 100% is 250 cd/m2, the contrast ratio is calculated as 500 by the equation 1.
When applying a voltage to the liquid crystal panel to change the minimum brightness voltage to Voffset2 and to maintain the G width, the screen brightness varies within the range from 0% to 90%. When it is assumed that the screen brightness of 0% is 0.5 cd/m2 and the screen brightness of 90% is 250 cd/m2, the contrast ratio becomes 450.
That is, when changing Voffest voltage and maintaining the voltage change width, the contrast ratio is reduced from 500 to 450.
FIGS. 2A and 2B are block diagrams showing apparatuses for varying the contrast ratio.
As shown in FIGS. 2A and 2B, when a data input device inputs R, G, B digital data to a scaler device as a vertical synchronous signal and a horizontal synchronous signal, the line and frame butter of the scaler device change a contrast offset voltage of FIG. 1 to change the contrast ratio. Then the scaler device scales the changed digital data which are suited to a liquid crystal display device module and sends the scaled digital data to a data output device. The data output device outputs the changed R, G, B data to a driver.
The driving method of the line buffer and the frame buffer are identical with each other. However, the data transforming method of the line buffer differs from that of the frame buffer only in that the line buffer transforms data in lines and the frame buffer transforms data in frames.
Both of the line buffer and the frame buffer are a memory device, which adjusts the contrast ratio using a hardware-like method. Due the use of the memory device, manufacturing cost increases. Furthermore, since the contrast ratio is fixed by the memory device, it is difficult to adjust the contrast ratio.